ABSTRACT
Vitamin A (all-trans-retinol, vitamin A alcohol) is the parent molecule of a family of compounds with shared biological activities. Collectively, these compounds are essential for visual functions and for the maintenance of healthy epithelial tissues (skin, immune system organs, gastrointestinal tract, reproductive organs, lungs, and others). The nutritional term vitamin A encompasses both preformed vitamin A and provitamin A. Preformed vitamin A (retinol and its esters) occurs naturally in foods of animal origin and is added in the preparation of some fortified foods and nutritional supplements. Provitamin A compounds (b-carotene, a-carotene, and b-cyptoxanthin), which are synthesized by plants as accessory photosynthetic pigments, can be metabolized by humans and animals to retinol. The essential metabolites of vitamin A include 11-cis-retinaldehyde, the form required for vision, andalltrans-retinoic acid (RA), which functions as a hormone and an essential regulator of gene transcription.
